This invention relates to radiation imaging using a semiconductor imaging device consisting of an array of image cells.
This invention describes a semiconductor imaging device for radiation imaging. The imaging device is an array of image cells, which consists of an array of radiation detector cells and an array of image cell circuits. An example of an imaging system configuration is shown in FIG. 1 of the accompanying drawings. All cells in the detector cell array are connected to respective electronics cells in the array of image cell circuits. With appropriate processing technology, it is possible to implement both detector cells and circuit cells on the same substrate. Another possibility is to have two substrates, one for the detector and one for the cell circuits and, by using a bump-bonding or other technique connect them mechanically and electrically together so that each detector cell is connected to the corresponding cell circuit. A cross-section of a part of an imaging device made of two substrates, which are bump-bonded together, is shown in FIG. 2 of the accompanying drawings.
In many radiation imaging applications, a need for different image resolutions exist. In single exposure images, the resolution should usually be relatively high. On the other hand, the same imaging system could be used for displaying live image by continuously reading the image from the imaging device and updating the display in real time. However, if the imaging system is designed for high resolution, the data bandwidth for a live image at, for example, 30 frames per second may be so high that the requirements for the readout electronics for handling the data stream may become unreasonable. A readout system fast enough to capture and process the images could become unreasonably expensive compared to the total cost of the imaging system. Furthermore, a high image resolution required for single exposure images may not even be required for a live display of images.
Therefore, a method for effectively reducing the resolution and thus the data bandwidth on chip would solve the problem. Another problem is the scalability of the imaging system for large or small area imaging systems. If single imaging devices with relatively small area could be easily linked together to form a seamlessly connected array of imaging devices for large area imaging system, the same imaging devices could easily be used for either large and small area applications.
This invention tries to solve the problems addressed above by introducing an imaging device with programmable image resolution and simple tiling of the devices to make a flexible imaging system for wide variety of target applications.
Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from dependent claims may be combined with those of the independent claims in any appropriate manner and not merely in the specific combinations enumerated in the claims.
In accordance with one aspect of the invention there is provided, an imaging device for radiation imaging, said device comprising an array of detector cells for generating a charge in response to incident radiation, an array of cell circuits for accumulating charge generated, and control circuitry controlling output of signals from said cell circuits, said control circuit operable to select an individual column and row of said array of cell circuits.
The array of detector cells and array of cell circuits form an array of pixels. As a result of the control circuits being operable to controllably select separate columns and rows, the resolution of the pixel array can be programmed.
In a preferred embodiment, the control circuitry is further operable to output separate enable signals for selecting said individual column and row. Thus providing individual selectability of columns and rows.
In a preferred embodiment, the said control circuitry is further operable to output a signal indicative of an end of row and a signal indicative of an end of column for said imaging device, thereby facilitating coupling separate devices together to form a large area detector.
In a preferred embodiment, the end of row and end of column output signals of a first device are connected to corresponding enable signals of an adjacent device in first and second orthogonal directions, respectively, to form an array of imaging devices for larger area radiation imaging.
In another aspect of the invention there is provided an imaging system, comprising a plurality of imaging devices according to any preceding claim connected as a one or two-dimensional array.
In a preferred embodiment the imaging system comprises a plurality of imaging devices according to any preceding Claim connected as a two-dimensional array, whereby said imaging system provides selectable imaging resolutions for selected applications.
Suitably, the control circuitry is arranged to permit reading of cell circuits of one row of pixels across multiple imaging devices from the two-dimensional array of imaging devices, before proceeding to a subsequent row.
In an embodiment of the invention, there is provided an imaging device for radiation imaging, the device comprising an array of detector cells for generating a charge in response to incident radiation, an array of cell circuits for accumulating charge generated, and control circuitry controlling output of signals from the cell circuits programmable to adjust the resolution of the imaging device.
As a result of the programmable resolution, an imaging device according to an embodiment of the invention can provide different operational modes giving different pixel resolutions for different target applications.
In a preferred embodiment, the programmability in that the control circuitry is arranged to select a group of cell circuits and to produce an output signal representative of a sum of charge accumulated in all cell circuits in a group. Thus, the control circuitry enables grouping of several pixels together to form a larger area super pixel for lower resolution imaging.
In a preferred embodiment the control circuitry averages signals representative of charge accumulated in all cell circuits in a group. For example, the output signal is representative of the total charge for all of the cell circuits of a group divided by the number of cell circuits in the group. Preferably, the number of cell circuits in a group is selectable from a set of possible numbers.
In a preferred embodiment, the output signal representative of charge accumulated is a current value. The use of a current output facilitates circuitry required to combine and average signal levels.
The control circuitry, for selecting a group of cell circuits, comprises a shift register arranged to select a plurality of columns or rows concurrently and to advance in steps of more than one row or column. The control circuitry can additionally comprise logic arranged simultaneous to select a plurality of rows and columns and a step size larger than one.
The control circuitry is arranged to average currents from a group of cell circuits by connecting current outputs of each cell circuit into a common output node and dividing the resulting sum of currents by the number of pixels in the group using a current mirror. The common output node can hold a current of the selected cell circuit(s).
Alternatively, for implementing group modes, each cell circuits in a group can be arranged to produce a scaled output signal representative of charge accumulated in the cell circuit divided by a number of cell circuits in the group. In order to be operable in a plurality of group modes, where each group mode has associated with it a predetermined number of cell circuits, the cell circuits can be arranged to include an output transistor for each group mode, which output transistor produces a scaled output signal according to the number of cells in a selected group mode. The output signal from all cell circuits in the group can then be averaged by summing the signals together.
In an embodiment of the invention, the resolution is controlled from outside by one or more control signals. For example, with two control signals, four different modes for resolution can be achieved. Thus, separate enabling signals can be provided for selecting columns and rows and output signals for indicating end of row or end of column.
Thus, in an embodiment of the invention, in addition to a mode where every individual pixel is read, 2xc3x972, 3xc3x973 or 4xc3x974 pixels could be grouped together and read out as super pixels. Other pixel combinations (for example having different numbers of rows and columns) and different number of modes can be used as well. The summation of pixel values can be easily done since the summation is done in current mode. Output currents of several cells are connected together. Adding currents from several cells together results in larger overall current. This can be compensated by an additional current mirror, which scales the current output to the same range as the current output of a single cell. In other words, the current mirror divides the current from a super pixel by the number of cells in the super pixel. This is equivalent to taking an average of a larger number of individual pixels. Using current mode output also has another advantage, enabling longer wiring without losing accuracy. Performing the averaging of pixel values is by no means limited to using current output. Voltage mode can be used instead of the current mode as described hereinafter. Using voltage mode would require the voltages of several pixels to be summed and averaged by using, for example, an op-amp circuit.
Moreover, an embodiment of the invention thus provides a solution to the problem of providing a video scan output from a imaging device constructed from a plurality of readout devices. Thus rather than reading out a device at a time, a large area imaging system formed from small area readout devices can be read one line from the whole imaging area before advancing to the next row of pixels. Together the readout devices form a seamless large area imaging system enabling scanned output over the whole image area. The imaging device has two input signals, which start the sequence for selecting the column and row for output. Furthermore, the imaging device has two outputs, one of which indicate when the last pixel of each line has been read and the other indicates when the last row of the device has been read. These output signals are connected to the corresponding input signals in the adjacent imaging devices in horizontal and vertical direction. The column and row output from the last imaging device can be connected to the first device to make the system run in a continuous mode for live video applications. The mode is selectable so that the user can switch between the single exposure mode and the live video mode at any time.
The combination of the above mentioned features makes it possible to use the same system for making single exposures with high resolution and at any time switching to live video mode and at the same time changing to lower resolution to reduce the data bandwidth. The size of the pixels is not fixed to any physical dimensions, but can be scaled according to available processing technology and based on the requirements of the target application.
The invention also provides an imaging system, comprising a plurality of imaging devices according to as defined above connected as a two-dimensional array, whereby the imaging system provides selectable imaging resolutions for selected applications. Control circuitry can permit reading of cell circuits one row at a time from the two-dimensional array of imaging devices, as opposed to one imaging device at a time.
In accordance with another embodiment of the invention, there is provided a method of operating an imaging device for radiation imaging, which device comprises of an array of detector cells for generating a charge in response to incident radiation, an array of cell circuits for accumulating charge generated, and control circuitry controlling output of signals from the cell circuits, the method comprising:
selecting a resolution of the imaging device;
adjusting addressing of the cell circuits to group outputs from the cell circuits according to a selected resolution.